docs(migration): note ssr-only module reload (#18755)

Author: bluwy

docs/guide/migration.md

@@ -101,6 +101,53 @@ There are other breaking changes which only affect few users.
   - This opt-in optimization was removed due to edge cases when writing a file in a cached folder and immediately importing it.
 - [[#18697] fix(deps)!: update dependency dotenv-expand to v12](https://github.com/vitejs/vite/pull/18697)
   - Variables used in interpolation should be declared before the interpolation now. For more details, see [the `dotenv-expand` changelog](https://github.com/motdotla/dotenv-expand/blob/v12.0.1/CHANGELOG.md#1200-2024-11-16).
+- [[#16471] feat: v6 - Environment API](https://github.com/vitejs/vite/pull/16471)
+
+  - Updates to an SSR-only module no longer triggers a full page reload in the client. To return to the previous behaviour, a custom Vite plugin can be used:
+    <details>
+    <summary>Click to expand example</summary>
+
+    ```ts twoslash
+    import type { Plugin, EnvironmentModuleNode } from 'vite'
+
+    function hmrReload(): Plugin {
+      return {
+        name: 'hmr-reload',
+        enforce: 'post',
+        hotUpdate: {
+          order: 'post',
+          handler({ modules, server, timestamp }) {
+            if (this.environment.name !== 'ssr') return
+
+            let hasSsrOnlyModules = false
+
+            const invalidatedModules = new Set<EnvironmentModuleNode>()
+            for (const mod of modules) {
+              if (mod.id == null) continue
+              const clientModule =
+                server.environments.client.moduleGraph.getModuleById(mod.id)
+              if (clientModule != null) continue
+
+              this.environment.moduleGraph.invalidateModule(
+                mod,
+                invalidatedModules,
+                timestamp,
+                true,
+              )
+              hasSsrOnlyModules = true
+            }
+
+            if (hasSsrOnlyModules) {
+              server.ws.send({ type: 'full-reload' })
+              return []
+            }
+          },
+        },
+      }
+    }
+    ```
+
+    </details>
 
 ## Migration from v4